In order for T cells to respond to foreign proteins, two signals must be provided by antigen-presenting cells (APCs) to resting T lymphocytes (Jenkins, M. and Schwartz, R. (1987) J. Exp. Med. 165, 302-319; Mueller, D. L., et al. (1990) J. Immunol. 144, 3701-3709). The first signal, which confers specificity to the immune response, is transduced via the T cell receptor (TCR) following recognition of foreign antigenic peptide presented in the context of the major histocompatibility complex (MHC). Polyclonal activators (e.g., anti-CD3 antibodies can also be used to transmit primary activation signals. The second signal, termed costimulation, induces T cells to proliferate and become functional (Lenschow et al. 1996. Annu. Rev. Immunol. 14:233). Costimulation is neither antigen-specific, nor MHC restricted and is thought to be provided by one or more distinct cell surface molecules expressed by APCs (Jenkins, M. K., et al. 1988 J. Immunol. 140, 3324-3330; Linsley, P. S., et al. 1991 J. Exp. Med. 173, 721-730; Gimmi, C. D., et al., 1991 Proc. Natl. Acad. Sci. USA. 88, 6575-6579; Young, J. W., et al. 1992 J. Clin. Invest. 90, 229-237; Koulova, L., et al. 1991 J. Exp. Med. 173, 759-762; Reiser, H., et al. 1992 Proc. Natl. Acad. Sci. USA. 89, 271-275; van-Seventer, G. A., et al. (1990) J. Immunol. 144, 4579-4586; LaSalle, J. M., et al., 1991 J. Immunol. 147, 774-80; Dustin, M. I., et al., 1989 J. Exp. Med. 169, 503; Armitage, R. J., et al. 1992 Nature 357, 80-82; Liu, Y., et al. 1992 J. Exp. Med. 175, 437-445).
The CD80 (B7-1) and CD86 (B7-2) proteins, expressed on APCs, are critical costimulatory molecules (Freeman et al. 1991. J. Exp. Med. 174:625; Freeman et al. 1989 J. Immunol. 143:2714; Azuma et al. 1993 Nature 366:76; Freeman et al. 1993. Science 262:909). B7-2 appears to play a predominant role during primary immune responses, while B7-1, which is upregulated later in the course of an immune response, may be important in prolonging primary T cell responses or costimulating secondary T cell responses (Bluestone. 1995. Immunity. 2:555).
One ligand to which B7-1 and B7-2 bind, CD28, is constitutively expressed on resting T cells and increases in expression after activation. After signaling through the T cell receptor, ligation of CD28 and transduction of a costimulatory signal induces T cells to proliferate and secrete IL-2 (Linsley, P. S., et al. 1991 J. Exp. Med. 173, 721-730; Gimmi, C. D., et al. 1991 Proc. Natl. Acad. Sci. USA. 88, 6575-6579; June, C. H., et al. 1990 Immunol. Today. 11, 211-6; Harding, F. A., et al. 1992 Nature. 356, 607-609). A second ligand, termed CTLA4 (CD152) is homologous to CD28 but is not expressed on resting T cells and appears following T cell activation (Brunet, J. F., et al., 1987 Nature 328, 267-270). CTLA4 appears to be critical in negative regulation of T cell responses (Waterhouse et al. 1995. Science 270:985). Blockade of CTLA4 has been found to remove inhibitory signals, while aggregation of CTLA4 has been found to provide inhibitory signals that downregulate T cell responses (Allison and Krummel. 1995. Science 270:932). The B7 molecules have a higher affinity for CTLA4 than for CD28 (Linsley, P. S., et al., 1991 J. Exp. Med. 174, 561-569) and B7-1 and B7-2 have been found to bind to distinct regions of the CTLA4 molecule and have different kinetics of binding to CTLA4 (Linsley et al. 1994. Immunity. 1:793). A new molecule related to CD28 and CTLA4, ICOS, has been identified (Hutloff et al. 1999. Nature. 397:263; WO 98/38216).
The importance of the B7:CD28/CTLA4 costimulatory pathway has been demonstrated in vitro and in several in vivo model systems. Blockade of this costimulatory pathway results in the development of antigen specific tolerance in murine and human systems (Harding, F. A., et al. (1992) Nature. 356, 607-609; Lenschow, D. J., et al. (1992) Science. 257, 789-792; Turka, L. A., et al. (1992) Proc. Natl. Acad. Sci. USA. 89, 11102-11105; Gimmi, C. D., et al. (1993) Proc. Natl. Acad. Sci USA 90, 6586-6590; Boussiotis, V., et al. (1993) J. Exp. Med. 178, 1753-1763). Conversely, expression of B7 by B7 negative murine tumor cells induces T-cell mediated specific immunity accompanied by tumor rejection and long lasting protection to tumor challenge (Chen, L., et al. (1992) Cell 71, 1093-1102; Townsend, S. E. and Allison, J. P. (1993) Science 259, 368-370; Baskar, S., et al. (1993) Proc. Natl. Acad. Sci. 90, 5687-5690. ).
Despite the structural similarities and shared affinity for the ligands B7-1 (CD80) and B7-2 (CD86) it is now clear that CD28 and CTLA-4 (CD152) and mediate essentially opposing effects on T cell activation. While the CD28/B7 interaction is known to serve as a positive co-stimulator in the context of TCR engagement by MHC/antigen complex, CTLA-4/B7 is now recognized as imposing a negative effect on cell cycle progression, IL-2 production, and proliferation of T cells following activation.
The development of novel methods for modulating the activities of CD28 and/or CTLA4 would be of great benefit in modulating the immune response. In addition, owing to the opposing effects of engagement of CD28 and CTLA4, specific compositions and methods for separately manipulating one or the other molecule on T cells would be beneficial. In particular, methods of specifically downmodulating T cell responses by modulating the CD28 pathway, while leaving the downmodulatory CTLA4 pathway intact would be beneficial in suppressing immune responses.